After reviewing 1,500 preliminary breath test (PBT) devices and more than 17.7 million tests conducted over five years it found more than 258,000 PBTs, or 1.5 per cent of all tests, had been falsified.

Professional Standards Command assistant commissioner Russell Barrett said the TAC had put $4 million of road safety funding on hold.

It is believed the fake tests were largely made by general duties and highway patrol officers, particularly in rural areas.

There is no suggestion officers misused breath testing devices while working at supervised drug and alcohol buses.

Mr Barrett saidformer police chief commissioner Neil Comriewould be appointed to look into the issue further.

"We're going to ask why this has occurred so we can learn from that and prevent repetitions of it and like practices in the future," he said.

Mr Barrett said Victoria Police was working with the TAC to "give them assurance".

"Our reputation's tarnished in the eyes of TAC and in the Victorian community. We can't walk away from that," he said.

TAC chief executive Joe Calafiore said he was supportive of the investigation to "get to the bottom of it".

"I think anything that has the potential to undermine confidence in what we do in road safety is a concern, and I share police's disappointment in what we've seen," he said.

Officers may have made false tests to 'highlight productivity'

Mr Barrett said he believed officers may have falsified the tests to meet targets.

"The question we all asked was why? There could be a number of reasons but the main rationale I believe is to hide or highlight productivity. Whatever reason our workforce may come up with, it isn't acceptable," he said.

He said there was no evidence to suggest officers had committed fraud, or any criminality has occurred.

"Similarly, there is nothing to suggest that any of this activity has impacted on any prosecutions," he said.

Mr Barrett said more oversight of preliminary breath testing would be put in place.